Spectral sensitivity functions derived from brightness matching: implication of intensity invariance for color-vision models

It is demonstrated that brightness sensitivity functions are self-similar; i.e., unique up to multiplication by a positive constant. This invariance leads to a structural restriction for sensitivity models that are formulated in terms of action spectra of cone systems. The heuristic value of this restriction is demonstrated through simulations on various kinds of data on direct brightness matching and brightness magnitude estimation on the basis of a new model for brightness sensitivity that accounts for both superadditivity and subadditivity.